Codec (coder-decoder) networks of a PCM data communications system use built-in filters which include an anti-alias filter designed to eliminate the loop distortion in a circuit stage posterior to the analog input of the coder network and a bandpass filter. Each of such anti-alias and bandpass filters is provided typically in the form of a switched capacitor filter and is thus required to be driven at a clock frequency of 1.024 MHz. This clock frequency is produced with use of a frequency divider which is designed to receive input clock pulses of a selected one of different standardized clock frequencies and to multiply each of the input clock frequency by a factor predetermined for the particular input clock frequency. When the T-1 PCM transmission clock frequency of 1.536 MHz or 1.544 MHz is to be used as the input clock frequency, the frequency divider multiplies the input frequency of 1.536 MHz or 1.544 MHz by the factor of 2/3 to produce the desired clock frequency of 1.024 MHz. When, on the other hand, the frequency divider receives an input clock frequency of 2.048 MHz in accordance with the CCITT Recommendation standard for the PCM transmission, then the frequency divider multiplies the input clock frequency by the factor of 1/2 for producing the clock frequency of 1.024 MHz for the switched capacitor filter.
Various types of clock frequency divider circuits have thus far been proposed and put to use to accomplish such selective division of the standardized clock frequencies. A frequency divider circuit of any of these types is however rather intricate in construction requiring the use of numerous circuit elements which add to the space to be occupied by the divider circuit when the divider circuit is implemented in the form of an LSI semiconductor chip.
It is, accordingly, an object of the present invention to provide an improved clock frequency divider circuit which is simple in construction using a minimized number of circuit elements which will occupy a significantly reduced area on an LSI semiconductor chip.